الفهرس 
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Abstract
S. maltophilia is an emerging multidrug resistant organism, it is an environmental pathogen which associated commonly with respiratory infections and other serious infections. S. maltophilia considered nowadays as the third most common non-fermenting Gram-negative bacilli that responsible for healthcare-associated infections.
S. maltophilia in human mostly cause colonization rather than infection. The relationship between host immune system and the organism is important because it is an opportunistic pathogen. It can adhere to medical material and to any indwelling catheters where it forms biofilm which facilitate progression of infection.
S. maltophilia is known to be resistant to a wide range of antibiotics due to acquired and intrinsic resistance, Trimethoprim/sulfamethoxazole (TMP/SMX) is the first line of treatment and the most effective antimicrobial agent in case of S. maltophilia even in resistant cases we can use (TMP/SMX) in combination with other drugs. TMP-SMX is a mixture of two antimicrobial drugs, which act synergistically and block microbial synthesis of folic acid, so it has bactericidal effect. Other drugs can be used such as levofloxacin, cephalosporins such as ceftazidime, ticarcillin-clavulanate, fluoroquinolones and tetracycline.
A lot of studies reported increased incidence of resistance to TMP-SMX which is mediated by dihydropteroate synthase (encoded by a sul genes) and dihydrofolate reductase (encoded by a dhfr or dfrA genes), sul1 and dfrA are carried on class1 integron encoded by Int1 gene while sul2 is present in plasmid carried on ISCR.
The aim of this work was to detect the genes encoding resistance to Trimethoprim/ Sulfamethoxazole among clinical isolates of Stenotrophomonas maltophilia.
This study was carried out during a period of one year and half (from June 2018 to December 2019). During this period a total of 100 clinical isolates of S. maltophilia, collected from different types of clinical specimens from microbiological labs of Mabaret Elasafra, Alexandria Main University Hospital and Medical Research Institute Microbiological lab.
Identification of S. maltophilia was done by both conventional methods, including morphology, culture characteristics, oxidase test, motility test and DNase test. S. maltophilia identification was confirmed by VITEK2.
Susceptibility of S. maltophilia isolates to different antibiotics was determined by Kirby-Bauer disk diffusion method according to CLSI recommendations 2019, Susceptibility of S. maltophilia to SXT was repeated by automated MIC. Conventional PCR was used to detect (sul, sul2, dfrA, Int1 and ISCR) for SXT resistance.
The majority of S. maltophilia isolates were isolated from respiratory samples (40%); (sputum (27%), Minibal (9%) and Bal4(4%)) followed by blood samples (38%), then by swabs 22 (22%).
The highest resistance among the isolates was to Ceftazidime 32(32%) followed by Ticarcillin/clavulanate 24(24%), Tetracycline 12(12%), 6(6%) Levofloxacin, while only 4(4%) of isolates were resistant to Minocycline. As regards to Trimethoprim-sulfamethoxazole; 16(16%) of isolates exhibited resistance by MIC method.
As regards SXT resistant genes. The sul1 gene was detected in (31/100 isolates), sul2 in (6/100 isolates) and dfrA that was found in only two isolates. On the other hand, Int1 gene that was detected in (32/100) of isolates and ISCR gene in (16/100 isolates).
Among the 16 TMP-SMX resistant S. maltophilia isolates that we analyzed, all of them possessed the sul1 gene and Int1 gene, while Sul2 gene was detected in 6/16(37.5%) TMP-SMX resistant isolates. The dfrA gene was detected only in 2/16(12.5%) TMP-SMX resistant isolates. ISCR gene elements were detected in 10/16(62.5%) TMP-SMX resistant isolates.
On the other hand, Among the 84 TMP-SMX sensitive isolates, sul1 gene was detected in 15 of the sensitive isolates (17.8%). While, int1 was detected in 16 of the sensitive isolates (19%) and ISCR gene elements were detected in 6 of the sensitive isolates (7.1%). None of the TMP/SMX-susceptible S. maltophilia isolates yielded positive sul2 or dfrA PCR products.